Hi Bill
Bill Payne wrote:
> On Sat, 06 Jan 2001 22:38:59 +1100 Jonathan Clarke
>
> Dear Jonathan,
>
> Thank you so much for your response. I had thought I might receive
> something like yours, and I appreciate it.
>
> I did my graduate work at the University of Tennessee and mapped a quad
> which included the Knox Dolomite at the base of the section, and a number
> of other carbonates above the Knox. Although I wasn't looking for karst
> at the contacts, it is my general recollection that the only karst I saw
> was on the outcrops which had weathered for years. I do know that there
> are plenty of carbonate formations here in Birmingham and north in
> Alabama which display planar bedding and a planar contact with overlying
> formations. In fact, I don't recall ever having seen any karst
> weathering except at the ground surface or in caves.
These features can be subtle in cross section. Mussman et al. noted that
that the Knox surface is a was in some places a disconformity, missing only a
few m The equivalent post-Romaine surface in Quebec is extremely planar for
the most part. However, it still shows features of subaerial dissolution, as
described by Desrochers and James.
>
>
> On page 208 of _The Genesis Flood_ by Whitcomb and Morris, there is a
> photo of Jeffersonville Limestone (lower Middle Devonian age) overlying
> Louisville Limestone (Milddle Silurian age). The photo caption says:
> Figure 23. A "Deceptive Conformity," or Paraconformity. This is a
> typical example of an extremely common, yet quite paradoxical,
> phenomenon, namely the perfectly conformable superposition of a younger
> bed upon a much older bed, with many intervening geological ages entirely
> missing. The Jeffersonville limestone, of lower Middle Devonian age, is
> here resting quite normally upon the Loiuisville limestone, of Middle
> Silurian age. The significant thing is that these formations are
> separated by more than 3000 feet of strata in other parts of the
> Appalachian trough, and therefore it must be assumed that many millions
> of years elapsed between them, although they look as though they must
> have been laid out in quick succession. This phenomenon has been
> variously called a "disconformity," a "deceptive conformity," and, more
> recently, by C. O. Dunbar and John Rodgers, a "paraconformity."
> (Principles of Stratigraphy, New York, Wiley, 1957, p. 119).
>
> Also, on page 210 of _The Genesis Flood_ is "Figure 24. Double
> Paraconformity." The caption says: In this Tennessee quarry are exposed
> two major paraconformities, above and below the Pegram limestone, which
> is lower Middle Devonian. The Chattanooga shale above is upper Devonian
> and the Lego limestone below is Middle Silurian. Again there is no
> physical indication whatever of any substantial time lapse btween the
> deposition of these various strata.
>
> Do you have any explanation for the _lack_ of karst weathering in the
> formations described above?
I did a quick search using Georf and material in the AGSO library, but
uncovered little that dealt with the contact relationships of these units.
The exception was one paper which did mention that there was quite
significant relief along the contact Jefferson and Lousiville Limestones, but
did not clarify whether this was depositional relief (the Louisville contains
numerous reefs), post depositional erosion, or some combination of these.
However the contract regionally is not as planar as the photos in Whitcomb
and Morris would suggest.
I should mention that there are also a number of other erosional processes
other than karst that effect limestones. We should not expect to find
evidence of karst along every limestone contact, although one should look for
it, as its presence is very important from understanding ancient environments
and for petroleum prospectivity.
respectfully
Jon
>
>
> Bill
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